Which of these numbers is prime? ${15,\ 24,\ 51,\ 83,\ 99}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 24 are 1, 2, 3, 4, 6, 8, 12, and 24. The factors of 51 are 1, 3, 17, and 51. The factors of 83 are 1 and 83. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 83 is a prime number.